The document describes how to locate an earthquake's epicenter using data from three seismograph stations. For each station, the difference in arrival times between the P and S waves is calculated and plotted on a travel-time graph to determine the distance to the epicenter. Triangulation is then used to pinpoint the exact epicenter location where the distance circles from each station intersect.

1. Locating the Epicenter
The source of an earthquake is called the focus,
which is an exact location within Earth were
seismic waves are generated by sudden release of
stored elastic energy. The epicenter is the point on
the surface of Earth directly above the focus. This
is the location that scientist calculate. The focus is
directly below, however, scientist can not determine
its depth.

To locate the position of an earthquakes
epicenter, we need a seismogram reading
from at least three different seismograph
stations and a travel - time graph which
shows the speed of both P- and S-waves.


2. Locating the Epicenter
Find the difference in arrival time between the P-wave and
the S-wave.

To do this refer to each of the three seismogram readings and
record arrival times for P-wave and S-wave.

Step 1: 3 Seismogram Records
P-Waves S-Waves
Arrival time
of P-waves
(23 Minutes)
Arrival time
of S-waves
(27 Minutes)
One example is shown below:

3. Locating the Epicenter
Subtract the arrival time of the S-wave from the arrival time
of the P-wave.

27 minutes – 23 minutes = 4 minutes
Step 2: Record Difference in Arrival Times
Thus, the S-wave arrived 4 minutes after the P-wave.
Next you plot 4 minutes on the
travel-time graph.


4. Locating the Epicenter
Subtract the arrival time of the S-wave from the arrival time
of the P-wave.

27 minutes – 23 minutes = 4 minutes
Step 2: Record Difference in Arrival Times
Thus, the S-wave arrived 4 minutes after the P-wave.
Next you plot 4 minutes on the
travel-time graph.

Slide the red line representing
4 minutes up between the P-wave
and S-wave lines until it fits
between the two lines.


5. Locating the Epicenter
Subtract the arrival time of the P-wave from the arrival time
of the S-wave.

27 minutes – 23 minutes = 4 minutes
Step 2: Record Difference in Arrival Times
Thus, the S-wave arrived 4 minutes after the P-wave.
Next you plot 4 minutes on the
travel-time graph.

Slide the red line representing
4 minutes up between the P-wave
and S-wave lines until it fits
between the two lines.

Read off the distance below.
Distance = 2500 km.


6. Locating the Epicenter
You need to do the same procedure
for two other seismograph stations.

Step 3: Record Distance to the Epicenter:
The S-P interval tells us the distance to the epicenter from each
seismograph station where the earthquake was recorded.

The epicenter of the earthquake is
located 2500 kilometers from the
first seismograph station.

Seismograph station 1 = 2500 km
Assume that the other readings are:

3500 km 4500 km
Seismograph station 2 = 3500 km
Seismograph station 3 = 4500km

7. Locating the Epicenter
The distance to the epicenter from each seismograph station is;
Station #1 -- d1 = 2500km
Station #2 -- d2 = 3500km
Station #3 -- d3 = 4500km

Step 4: Use Triangulation to Pin Point the Epicenter:
At each station we can draw a
circle on a map that has a
radius equal to the distance to
the epicenter from each
seismograph station.

Three such circles will intersect
in a point that locates the
epicenter of the earthquake.


8. Sample Problem
The table below provides information collected at seismic stations A,
B, and C for the same earthquake.
Answer:
(i) Why are no S - waves recorded at station A?
No S-waves are recorded at station “A” because station
“A” is located in a shadow zone, an area 103E - 143E
from the earthquake epicenter.
Seismic
Station
P - Wave Arrival
Time
S - Wave Arrival
Time
Difference in
Arrival Times
Distance to
Epicenter(km)
A 08:48:30 No S - waves
B 08:42:00 00:04:30
C 08:46:00 3500
Seismic
Station
P - Wave Arrival
Time
S - Wave Arrival
Time
Difference in
Arrival Times
Distance to
Epicenter(km)
A 08:48:30 No S - waves
B 08:42:00 00:04:30
C 08:46:00 35002000

9. Sample Problem
The table below provides information collected at seismic stations A,
B, and C for the same earthquake.
Answer:
(ii) What is the distance from station B to the epicenter?
The difference in arrival time between P-wave and S-wave is 4 min.
and 30 sec. (00:04:30). Graph this on the time scale (5 blocks) and
find the position between the P- and S-wave lines. Then read off the
distance on the x-axis as shown in the diagram.
Seismic
Station
P - Wave Arrival
Time
S - Wave Arrival
Time
Difference in
Arrival Times
Distance to
Epicenter(km)
A 08:48:30 No S - waves
B 08:42:00 00:04:30
C 08:46:00 3500
Seismic
Station
P - Wave Arrival
Time
S - Wave Arrival
Time
Difference in
Arrival Times
Distance to
Epicenter(km)
A 08:48:30 No S - waves
B 08:42:00 00:04:30
C 08:46:00 35002000

10. Sample Problem
The table below provides information collected at seismic stations A,
B, and C for the same earthquake.
Answer:
(iii) What is the arrival time of the S - wave for station C?
Locate 2000km on the diagram and draw a line straight up until you
intersect the S-wave line, (as indicated by blue arrow). Then draw a
line horizontally left to intersect the travel-time (y-axis) axis, and read
off the time (as indicated by the red arrow).
Arrival time of the S-wave at 2000km would be 7 minutes.
Seismic
Station
P - Wave Arrival
Time
S - Wave Arrival
Time
Difference in
Arrival Times
Distance to
Epicenter(km)
A 08:48:30 No S - waves
B 08:42:00 00:04:30
C 08:46:00 3500
Seismic
Station
P - Wave Arrival
Time
S - Wave Arrival
Time
Difference in
Arrival Times
Distance to
Epicenter(km)
A 08:48:30 No S - waves
B 08:42:00 00:04:30
C 08:46:00 35002000